feat: admin Text Gen tool — chapter names + book description via CF Workers AI

Adds backend handlers and SvelteKit UI for an admin text generation tool.
The tool lets admins propose and apply AI-generated chapter titles and book
descriptions using Cloudflare Workers AI (12 LLM models, model selector shared
across both tabs).

ui/messages/ru.json

@@ -363,6 +363,7 @@
   "admin_nav_translation": "Перевод",
   "admin_nav_changelog": "Изменения",
   "admin_nav_image_gen": "Image Gen",
+  "admin_nav_text_gen": "Text Gen",
   "admin_nav_feedback": "Отзывы",
   "admin_nav_errors": "Ошибки",
   "admin_nav_analytics": "Аналитика",